iio: srf04: add power management feature

Add suspend and resume operations for being used by optional power
management.

The suspend function is switching off an GPIO which can be used by the
hardware to switch power off. The resume function is switching the GPIO
on and sleeps an adjustable time to give the device a chance to be up
and running.

If activated the driver gets into autosuspend after some time of
inactivity.

Suggested-by: Franz Parzer <rpi-receiver@htl-steyr.ac.at>
Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>